1. Field of the Invention
The present invention relates to a method and system for determining weld quality in resistance welding, and more specifically to a method and system for determining weld quality in such a manner that acceptance or unacceptance is determined by comparisons with their respective reference values of the electrode voltage, the electrode current time, the number of the electrode voltage pulse signals in excess of a reference value, and the integral of the difference between the electrode voltage in excess of a base voltage and the base voltage, independently or simultaneously, in addition to the comparison of the electrode resistance with its reference value.
2. Description of the Prior Art
There are various techniques of ensuring weld quality without destroying the welded products during welding work, such as a welding current monitoring system, a welding energy monitoring system, an electrode tip resistance monitoring system, an electrode tip voltage monitoring system, an electrode tip potential monitoring system, and so on.
In the welding current monitoring system, although it is very effective in discovering power failure or disconnection of the secondary coil of the power transformer, since the welding current is monitored, it is not particularly effective in detecting deterioration of weld quality due to changes in the contact conditions between members to be welded, the welding current, the shape of the electrode tips, and so on.
In the welding energy monitoring system, although the total electrical energy supplied to the welded portions is monitored, since weld quality is determined not by the total energy but by the distribution of energy density with respect to time, it is virtually impossible to monitor welding quality when the area of the weld changes according to the weld conditions.
In the electrode tip resistance monitoring system, although the resistance between the electrode tips is detected and then the weld quality is monitored depending upon the detected resistance, since the resistance between the electrode tips impose a serious influence upon the area of the welding current path between the members to be welded, it is virtually impossible to determine the weld quality when the shape of the electrode tips is deformed or the electrodes are crushed so as to form other current path shapes.
In the electrode tip voltage monitoring system, the voltage between the electrode tips is detected and the weld quality is determined from consideration of the detected voltage. The electrode tip voltage has generally a close relationship to the rise in temperature of the weld, and to the rate of bead penetration especially in the case of spot welding.
In the case where the pressure applied to the electrodes is great and the members to be welded are thin, since the current path is formed according to the electrode pressure and the shape of the electrode tips, even if the electrode current changes because of the formation of other current paths, it is possible to determine the weld quality. However in the case where relatively thick members are welded, or where high-tension steel plates are welded, or where a relatively low electrode pressure is applied therebetween, or where press-manufactured products are welded, it is impossible to ensure the appropriate current path area between members to be welded (this current path area has a relationship to the bead area), and therefore there are some cases in which it is impossible to obtain the required bead diameter.
Further, in the case where the electrode tips are crushed, since a bead having a diameter greater than that required is obtained, it is impossible to determine the weld quality accurately.
In the electrode tip potential monitoring system, although it may be effective in determining weld quality, since the rate of detection is relatively slow when this system is applied to spot welding on site, in practice it is very difficult to measure the potential accurately because of noise.
As described hereinabove, in the conventional methods of determining weld quality in resistance welding there exist no desirable methods because of reasons such as that the welding factors are related to the weld quality only within a defined range, or have no direct relationship to the weld quality, or are insufficient to determine the weld quality.